After they have entered the horizon, the \(\Phi \) 
fluctuations oscillate as follows [1]}
\(\Phi (\vec{k},t) &=& 3 \Phi _p(\vec{k})\left({\sqrt{3} \over 2 k \sqrt{t t_1}}\right)^3\left[\sin \left(2 k \sqrt{t t_1 \over 3}\right)- {2 k \sqrt{t t_1 \over 3}}\cos \left(2 k \sqrt{t t_1 \over 3}\right)\right]\nonumber \\&=& - {9 \over 4 k^2 t t_1}\Phi _p(\vec{k})\cos \left(2 k \sqrt{t t_1 \over 3}\right)~~~~~{\rm for}~~k \sqrt{t t_1} >> 1 ~~\ .\) 
